For a long time it has been sought to minimise the influence of variations in external conditions, in particular temperature and magnetic field, on the isochronism of a sprung balance regulating system by acting on the construction and choice of materials.
As regards the balance spring, since the discovery of invar, which is an Fe—Ni alloy, at the beginning of the 20th century, more and more complex alloy compounds have been proposed in order to tend towards optimum quality. EP Patent No. 0 886 195 discloses for example Niobium and Zirconium alloy compounds further containing an interstitial doping agent formed at least in part of oxygen and allowing the CTE value to be controlled. Given the complexity of the compounds and the use of rare metals, the cost of the final product is high. Moreover, manufacturing a metallic balance spring relies on complex metallurgic processes (wire drawing, hardening, annealing, etc. . . . ) that do not guarantee perfect reproducibility of the desired resilient properties, such that it is still necessary to pair the balance and spring during assembly.
It has also been sought for a long time to replace an alloy by another material that does not have the drawbacks of metal. CH Patent No. 307 683, published in 1955 proposes fabricating a balance spring in glass with a silicon content such that the value of its expansion coefficient is practically zero without it being necessary to add any rare metal. This patent does not disclose how such a balance spring is made, nor what influence it has on the CTE.
A more recent document, EP Patent No. 1519250 discloses a sprung balance resonator that is thermocompensated owing to cut and structuring along determined orientations in relation to the crystallographic axes of a quartz crystal. Only qualified persons can implement such manufacturing. It will also be observed that the crystalline nature of quartz makes the balance spring brittle under shock causing fissures that propagate along the crystalline planes.